Mobs, aside from villagers/golems, I have noticed have been grouping together in hordes. I have seen as many as 20+ sheep all gathered at once, to a counted group of 16 creepers.

In most cases, I find a high number of mobs in groups of 4-7, but on rare occasions it gets really high. I could be wrong, this could be intentional. But that many creepers is like saying, "Here, take a nuking."